It is good practice to use version control to save a history of code changes you make as well as sharing code with collaborators. In ATLAS, we use git for version control.
An introduction to git is available here:
Note that the tutorial uses GitHub for remote repositories while ATLAS uses GitLab. The web interfaces between the two differ, but the core concepts are the same.
Make sure you are very comfortable with lessons 1-9. Feel free to work through the remaining lessons in your own time.